I my search for tips through e-mail and the internet, I have come across the following published by the Saskatoon Railroad Modelers, our brothers North of the border. I have re-written parts to "Americanize" it, since they do speak and write a little differently than we do. I have tried this on my Bachmann 1:20.3 Spectrum couplers with success.
Bachmann Large Scale couplers are sturdy, inexpensive, and even if oversize, are at least shaped about like the real thing. But they sometimes have a nasty habit of uncoupling at the wrong time. Occasionally just taking up the slack in the couplers is enough to cause one of them to open. The problem seems to be that the locking pin is too light and lifts too easily. Adding weight helps a bit on completely smooth track, but if the track has any bumps, the added weight often causes the coupler to open when the wheels hit a bump and the weight, along with the locking pin, are thrown upwards. Our solution is to add a spring that holds the pin down. Compared to a weight, the spring will not allow the pin to lift when hitting a bump. As an added bonus, the spring can be extended to form an easily operating cut lever. Installation requires about 5 minutes per coupler. The first step is to drill two holes in the coupler. Drill the hole at a point 1/8" forward of the rear of the body and 3/32" in from the side of the body. (You may have to drill in both directions) See the diagram below, keeping in mind that it shows the coupler from the top.
You can drill the holes with a #76 drill chucked in a Dremel tool, but a pin vice works well too. Drill the second hole horizontally through the locking pin. There is usually a dimple about 1/8" down from the top which makes it easy to start the drill there.
Next step is to bend a spring. We usually use .015 music wire from the hobby shop, but .020 works well too, although you may have to drill the holes a little larger if you do. Cut off a piece of wire 4 inches long or a bit longer. You can cut music wire with hardened diagonal cutters, or with an abrasive wheel in a Dremel tool. Starting at the mid point of your piece of wire, wrap it tightly around a 3/32" diameter rod - the shank of a 3/32" drill bit works fine. Wrap it for two full turns.
Your spring should now look like a spider with one short leg and one longer leg, with the legs parallel to each other. If the legs are not parallel, bend them until they are. Then bend the short leg 90 degrees outward at a point 1/4" from the top of the spiders head. Lastly cut the longer leg to make the spider 1-1/2" tall.
Thread the bent leg through the two aligned holes in the corner of the coupler body, working from the top down. When the eye of the spring (the spider's head) is still an inch or more above the coupler, thread the other leg through the locking pin. The spring will bend to accommodate this. With both ends inserted in their holes, push the spring eye down to the top of the coupler.
Finish off the installation by cutting off the extra length below the coupler Cut off the extra length, but leave 1/8" or so below the coupler. If you are using side cutters, put one finger over the spring eye to prevent it from being ejected when the cutters snap through the wire. Then bend the extra 1/8" at 90 degrees or a little more, bending it toward the front of the coupler. Lastly, bend over the last 1/4" of the cut lever. Slipping a piece of small heat shrink tubing over the cut lever stiffens the lever slightly and provides finger protection.
